1. Field of the Invention
The present invention relates generally to a machine used in a process of manufacturing an Inconel spacer grid which is one of the critical parts of a nuclear fuel assembly and, more particularly, to a machine for automatically grinding the outer surface or corner of the spacer grid to eliminate taps and remnants of filler metal which are formed on the outer surface or corner of the spacer grid.
2. Description of the Related Art
As is well known to those skilled in the art, a nuclear fuel assembly is an assembly for nuclear fuel elements and is configured such that, when the nuclear fuel elements are loaded into or extracted from a nuclear reactor, they are integrally handled as a single body rather than as individual components. For example, in a nuclear fuel assembly for a light-water reactor, a fuel rod is manufactured by sintering uranium oxide into a pellet shape and sealing the uranium oxide in a hollow rod which is made of a Zircaloy alloy and which has a length of about 4 m. A plurality of fuel rods, for example, in the case of a boiling water reactor, about sixty fuel rods, and in the case of a pressurized water reactor, about two hundred and thirty fuel rods, is formed into a square grid-shaped bundle. Support plates having holes through which a coolant passes are respectively mounted to the upper and lower ends of the bundle. A spacer grid is provided in a medial portion of the bundle to maintain distances among the fuel rods constant.
In the nuclear fuel assembly, a structure which serves as a frame into which fuel rods are inserted is called a frame body. The frame body includes a top nozzle, a bottom nozzle, a guide thimble, a measurement tube and spacer grids. These elements are assembled together by welding or a mechanical coupling method. As such, the frame body of the nuclear fuel assembly functions to maintain distances among the fuel rods constant and support the fuel rods. In addition, the frame body forms the basis of the mechanical structure of the nuclear fuel assembly.
In the frame body, each Inconel spacer grid is manufactured by assembling outer straps with inner straps into a grid shape and by brazing using filler metal and is processed by heat treatment. Therefore, the spacer grid is easily deformed during the manufacturing process, thus making it difficult to meet the standardized shape. Furthermore, the spacer grids are provided on the upper and lower portions of the frame body and thus support the entire frame body. In addition, the spacer grids function to maintain distances between nuclear fuel assemblies in the nuclear reactor. Therefore, the outer measurements of the spacer grid must be precise. Also, protrusions must not be present on the outer surface of the spacer grid after an operation of grinding is completed. When conducting the grinding, a depth to which the spacer grid is ground must be as shallow as possible to prevent the brazed portions from being damaged.
To meet the above-mentioned requirements, the Inconel spacer grid has a very complex shape. Hence, it is very difficult to grind the spacer grid using an automated machine. However, because the spacer grid serves a very important role in the nuclear reactor, it must be very precisely ground.
Accordingly, the operation of grinding the outer surface of the spacer grid requires very high precision and has been conducted by a manual method to ensure the quality of the product. However, the manual operation requires much time. Furthermore, taking into account the demand of Inconel spacer grids, improvement in productivity is required.
As such, in conventional arts, the grinding operation has been conducted by a manual method, because the shape of the Inconel spacer grid is not standardized and it is thus not easy to grind it using an automated machine. For example, in a conventional art, target portions to be ground were processed one by one using a hand grinder. However, in the case of such a conventional grinding method, there is a probability that a worker will contract a musculoskeletal disease. Furthermore, working surroundings are poor because of noise of the hand grinder and the dust that is generated when grinding, so that at least two workers should work in shifts of four hours each day.
Moreover, intensity of work is very high, reducing productivity. The manual work cannot make the ground portions uniform.
Therefore, automation of the grinding operation is required to improve the quality of the spacer grid, the productivity and the work surroundings as well as to prevent a worker from contracting a musculoskeletal disease.